In recent years, with an increase of communication traffics, an optical transport network for a trunk line system has been required to transport a larger amount of data per one optical fiber. As a means for realizing this, attention has been paid to a multilevel modulation technique and a digital coherent detection technique to realize an improved frequency efficiency and a long-distance transmission. In the case of the multilevel modulation, a sophisticated optical modulator must be realized in consideration of a light phase.
An optical modulator is a basic device for optical communication for converting an electric signal to a light intensity signal. For example, the optical modulator has been generally required to provide a high speed, low loss, low power consumption, a small size, and high reliability. The method of realizing an optical modulator is classified to a direct modulation scheme and an external modulation scheme. A high-speed backbone network mainly uses the external modulation method from the viewpoints of high speed and a long-distance transmission. An optical modulator using the external modulation scheme consists of dielectric material such as LiNbO3 (lithium niobate, hereinafter referred to as LN), semiconductor material or organic material with an electro-optic effect (hereinafter referred to as EO), and semiconductor material with an electroabsorption effect for example.
In the case of an optical modulator based on the multilevel modulation scheme, a passive optical circuit for multiplexing and demultiplexing polarization light is required because polarization light must be actively used. However, LN and semiconductor material have inferior optical characteristics to that of glass material from the viewpoints of low loss and connectivity to optical fibers, and then have a disadvantage in functional improvement.
As a device for realizing a passive optical circuit with a low loss, a planar lightwave circuit (hereinafter referred to as PLC) has been known in which silica glass is deposited on a Si substrate for example. Attention has been paid on a technique to use the superior optical characteristic of a silica-based PLC consisting of silica glass material to combine the silica-based PLC with dielectric material (e.g., LN) or an optical functional element consisting of semiconductor material or organic material for example.
In the case of the optical modulator as described above, an optical input/output section between a silica-based PLC chip and the optical functional element chip is appropriately connected and integrated. Two or more integrated chips are handled as one device (hereinafter a multi-chip integrated device) to connect optical fibers for performing the optical input/output to the exterior to the multi-chip integrated device. A typical example of the optical modulator using the multi-chip integrated device has been known as a modulator obtained by combining a silica-based PLC with an LN waveguide (hereinafter referred to as PLC-LN modulator).
Generally, an optical device such as an optical modulator is provided on a board in a communication equipment by being contained, from the viewpoints of reliability or a gas barrier property for example, in a package or a case consisting of metal or ceramic for example. Optical fibers and an optical device are generally adhered and fixed by a fiber connecting part consisting of glass for example. Optical fibers are connected to an optical device by penetrating through the pipe section of the package or case. Metal-coated fibers obtained by coating optical fibers with a metal coating are generally used to solder-seal the pipe section or to fix the optical fibers by adhesive agent for example.
In such an embodiment, optical fibers are fixed at two points of a connecting part of the optical device mounted in the package and a pipe section. Metal used for the package and glass material or semiconductor material used for the optical device have different thermal expansion coefficients. Thus, thermal stress depending on a temperature change causes tensile stress or compressive stress to the optical fibers, thus resulting in a changed position of the optical fibers. This has caused a disadvantage of reduced mechanical reliability and optical characteristic of the optical fibers themselves or the element for fixing the optical fibers.
In order to solve the above-described disadvantage, several researches have been made. For example, a structure has been suggested to buckle the optical fibers by a fixed length between the connecting part of the optical device and the pipe section in order to absorb the position variation (see for example Patent Literature 1). The multi-chip integrated device such as a PLC-LN modulator also has a similar structure in which the optical fibers are fixed at two points in the package. Two or more chips also have a difference in the thermal expansion coefficient, thus further deteriorating the disadvantageous thermal stress.
FIG. 1 illustrates the structure of a conventional PLC-LN modulator. A PLC-LN modulator 10 is configured so that an LN modulator 12 is contained in a package 11 in which both ends are connected to silica-based PLCs 13a and 13b. Optical fibers 14a and 14b are connected by fiber connecting parts 30a and 20b to silica-based PLCs 13a and 13b at connection end faces 21a and 21b and are fixed to pipe sections 22a and 22b of the package 11.
For example, these parts have thermal expansion coefficients (unit: ×10−6/K) as shown in the following table.
TABLE 1Thermal expansionPart namecoefficientStainless (SUS303)17.3Optical fibers0.75Fiber connecting part (glass)3.2Silica-based PLC2.5LN15.4
When stainless is used as package material in particular, a difference in thermal expansion coefficient between stainless and silica-based PLC is higher than that between a fiber connecting part consisting of glass and silica-based PLC. Thus, stress is concentrated on a connecting section between the optical fibers and a light waveguide on the silica-based PLC and a connecting section between the light waveguide on the silica-based PLC and the light waveguide of the LN modulator, thus resulting in a reduced mechanical reliability. Even when the LN modulator is compared with a package consisting of stainless, the former and the latter do not have a completely the same thermal expansion coefficient, resulting in the disadvantage of thermal stress unsolved.
In view of the above, the PLC-LN modulator 10 is configured so that the optical fibers are buckled between the two points for fixing the optical fibers 14a and 14b to absorb the above-described tensile stress or compressive stress due to thermal stress. Thus, the package interior requires fiber extra lengths 23a and 23b in order to achieve a fixed amount of buckling. For example, when the package 11 having a longitudinal direction of about 100 to 200 mm is used, the fiber extra lengths 23a and 23b must be about 8 to 15 mm, respectively.
Thus, in the case of the structure in which the optical fibers are buckled in the package, a space having a fixed length must be secured between the fiber connecting part and the pipe section, which may hinder a package size from being smaller. This distance can be reduced if the buckling amount of the optical fibers is increased. However, the increased buckling amount of the optical fibers has caused a disadvantage of an increased fiber bending loss or reduced reliability.